Catalysts for aromatic compound production or for reforming generally comprise a support (for example, formed from at least one refractory oxide, the support possibly also including one or more zeolites), at least one noble metal (preferably platinum), at least one promoter metal (for example tin or rhenium), at least one halogen and possibly one or more additional elements (such as alkalis, alkaline-earths, lanthanides, silicon, elements from group IVB, non noble metals, elements from group IIIA, etc. . . . ). Catalysts of that type contain, for example, platinum and at least one other metal deposited on a chlorinated alumina support. In general, those catalysts are used to convert naphthenic or paraffinic hydrocarbons which can be transformed by dehydrocyclisation and/or dehydrogenation; by reforming or for the production of aromatic hydrocarbons (for example the production of benzene, toluene, ortho-, meta- or para-xylenes). Those hydrocarbons originate from fractionation of crude oil by distillation or other transformation processes.
Such catalysts have been widely described in the literature.
Regeneration of such catalysts is also well known. It is carried out in a fixed or moving bed, with a combustion step carried out in one or more combustion zones, then an oxychlorination step, followed by a calcining step. European patent EP-A-0 378 482 describes such a moving bed process.
In order to facilitate comprehension, the process of the invention will be described with respect to moving bed regeneration of reforming catalysts. Reference should first be made to the prior art of EP-A-0 378 482.
According to EP-A-0 378 482, the used catalyst slowly moves from top to bottom of a regeneration vessel where it successively comes into contact with a first radial moving bed combustion zone, a second radial moving bed combustion zone, an axial oxychloration moving bed zone and an axial calcining moving bed zone; and:    a) in the first combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reforming reactor, at a temperature in the range 350° C. to 450° C., using a combustion gas based on an inert gas circulating radially in a counter-current to the catalyst, comprising 0.01% to 1% by volume of oxygen, the combustion gas originating from a zone for washing the gases from the calcining, oxychlorination and combustion zone;    b) in a second combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reactor, at a temperature which is at least 20° C. higher than the temperature in the first combustion zone, in the presence of gases originating from the first combustion zone and in the presence of an inert makeup gas to which up to 20% by volume of oxygen is added to that the catalyst is in contact with a gas comprising 0.01% to 1% by volume of oxygen, the gases circulating radially and in a co-current with the catalyst.
The catalyst is then sent to the oxychlorination zone.
We have established that when carrying out this regeneration process, it is important that combustion is terminated (complete) before passing to the oxychlorination step (or any other subsequent step). If not, the combustion conditions must be modified to achieve it.
As a result, we have developed a process for regenerating a catalyst, the process including a zone for monitoring and controlling combustion completion, before any subsequent step, using means for monitoring and controlling the end of combustion.
We are aware of U.S. Pat. No. 4,578,370 which describes a temperature adjusting zone, after the combustion step and before the oxychlorination step in a process for regenerating a reforming catalyst.
In that process, the gas supplied to the combustion zone circulates radially in the catalyst bed, and the oxygen for combustion originates essentially from the calcining zone located in the bottom of the regeneration zone, after the combustion zone.
The temperature adjusting zone modifies the temperature of the catalyst leaving the combustion zone to bring it to a temperature close to that in the oxychlorination zone, to reduce the thermal shock undergone by the catalyst as it enters the oxychlorination zone.
In order to achieve this, a gas is injected into the temperature adjusting zone, the gas being a compressed portion of the effluent from the combustion step.
That gas is at the temperature of the compressed effluent from the combustion step. It has neither been cooled nor re-heated since one of the aims of that process is not to use heating means, and in general to reduce the amount of equipment required.
The gas entering the temperature adjusting zone mixes at the bed outlet with the gas from the oxychlorination zone and rises, mixing with combustion fumes, up to the head of the regeneration vessel from which it is extracted.
When passing through the temperature adjusting zone, the gas heats the catalyst.
The present patent application proposes a process including a zone located after combustion has been carried out, but this zone can overcome a different problem (monitoring and controlling the end of combustion) using means which are not used in U.S. Pat. No. 4,578,370.
More precisely, the present invention concerns a process for regenerating a used catalyst including combustion of carbonized material, in at least one combustion step in which the catalyst is brought into contact with at least one oxygen-containing gas, under pre-determined operating conditions, the process being characterized in that the catalyst which has undergone all of the combustion steps is, in a monitoring and control step, brought into contact with at least one oxygen-containing gas, called monitoring and control gas, under conditions which are more severe than those of the combustion steps, and in that the monitoring and control step is carried out with an oxygen consumption of less than 10% of the oxygen entering with said gas.
The severity of the conditions in the monitoring and control zone is achieved by introducing by means of the monitoring and control gas, a quantity of oxygen which is at least equal to the largest quantity of oxygen introduced into a combustion step and/or by introducing the monitoring and control gas at a temperature which is higher than that of the gases introduced into the combustion step. The first method is preferred.
The monitoring and control step is advantageously carried out at a substantially constant temperature.
The term “substantially constant temperature” means a temperature which does not vary by more than 3%, preferably not more than 2%, excluding measuring errors and heat losses.
The monitoring and control gas temperature is advantageously at least equal to that of the highest temperature of the gas(es) introduced into the combustion step. It is preferably higher.
The temperature is preferably of the same order of magnitude as the temperature of the gases and/or the catalyst leaving the last combustion zone.
Monitoring and control is advantageously carried out on the monitoring and control gas leaving the process independently of the other gases from the combustion step after passage over the catalyst. It can also readily be carried out on a monitoring and control gas leaving as a mixture with at least one gas from at least one combustion step.
The used catalyst is regenerated, for example in a fixed bed or in a moving bed, with continuous or intermittent flow.